Current research into mechanisms of synaptogenesis is largely performed in vitro. An experimental system which preserves normal three-dimensional cellular relationships and in which synaptogenesis proceeds as in vivo is needed to test the relevance of in vitro findings and to explore aspects of synaptogenesis that depend on the cytoarchitecturee of intact tissues. Ideally, such a system will be one in which (1) pre- and postsynaptic elements can be identified and separately monitored and manipulated in a variety of ways throughout synaptogenesis; (2) the earliest nerve-muscle contacts and synapses can be identified individually in the light microscope (LM) and subsequently examined electron microscopically (EM); (3) the synapses are amenable to freeze-fracture and rapid-freezing; and (4) the normal pattern of growth and differentiation of pre- and postsynaptic elements has been well established. The purpose of the study proposed here is morphological characterization of such an experimental system: the tadpole rectus abdominis muscle and nerve. A combination of LM, histochemistry, freeze-fracture and EM will be used to study the formation and development of the muscle and its neuro-muscular junctions (NMJs) in vivo. As a baseline for future research into underlying synaptogenic mechanisms, changes in number, location, size and shape of NMJs will be mapped against a background account of changes in the size and number of muscle fibers. Developmental differences between twitch and slow junctions will be sought and the resolution of multiple innervation charted. The earliest aggregates of acetylcholine receptors detectable with rhodamine-labeled Alpha-bungarotoxin will be identified in the LM and subsequently examined by freeze-fracture to determine the form in which receptors first appear in the postsynaptic membrane. Freeze-fracture will also be used to study the development of specializations in the presynaptic membrane. A method will be developed to maintain the entire abdominal body wall, including the spinal cord, in vitro during the one- to three-day period when muscle cells and nerve processes align in the muscle site. In future experiments, such preparations will be manipulated mechanically and chemically to probe the mechanisms that determine where, when and how the muscle fibers align and the NMJs form.